1. Field of the Invention
The present invention relates generally to Shigella vaccine, strains, their use in vaccines, and the methods for treatment of dysentery.
2. Related Art
Shigella spp. is the causative agent of bacillary dysentery. The distal end of the colon and the rectum, which show intense and acute mucosal inflation, are the areas of the gastrointestinal (GIT) moat most affected during shigellosis. A very low inoculum of 10-100 bacteria is sufficient to cause the disease, which then spreads easily, often directly, by the fecal-oral route or by flies contaminating food and water. Appropriate antibiotics, such as those effective against gram-negative bacteria, are used to combat Shigella infection; however, as with most bacterial pathogens, an increase in antibiotic resistance has dramatically emphasized the need for a safe and effective vaccine.
Recent Shigella vaccine candidates include subcellular complexes purified from virulent cultures (e.g., Invaplex vaccine) that encompass detoxified lipopolysaccharide (LPS) conjugated to carrier proteins and live attenuated vaccine strains. Attempts to utilize whole-cell inactivated organisms that are given in high and multiple doses have met with poor immunogenicity and protective efficacy. Live vaccines appear to offer a better approach since they mimic natural infection while subverting the clinical outcome.
U.S. Pat. No. 5,762,941 illustrates one live vaccine approach and involves the loss of virG(icsA) gene in S. flexneri 2a, S. sonnei, and S. dysenteriae 1. Vaccine strains SC602, WRSS1, and WRSd1 resulted, of which SC602 and WRSS1 were tested in Phase 1 trials on human volunteers and found to be safe at oral doses ranging from 103 to 104 CFU. However, 15-25% of the volunteers administered SC602 or WRSS1 showed symptoms of mild diarrhea and fever.
U.S. Pat. No. 5,589,380 and U.S. Pat. No. 5,468,639 illustrate another approach, whereby two candidate genes, setAB(shET1) and senA(shET2), are utilized since they have previously been shown to encode enterotoxic activity. While setAB(shET1) is present predominantly on the chromosome of S. flexneri 2a strains, the senA(shET2) gene is located on the large virulence plasmid of all Shigella strains.
An article in the Journal of Infectious Disease 2004, 190: 1745-54 illustrates a still further approach to achieve a live Shigella vaccine. The symptoms of diarrhea and fever seen with a guaBA mutant of S. flexneri 2a vaccine candidate CVD1204 were ameliorated with the administration of CVD1208, which contains the guaBA mutation in combination with deletions in setAB and senA(shET2) genes. Notably, senB(shET2-2) constitutes a putative enterotoxin gene that shares >60% homology at the amino acid level with senA(shET2-1) and could be a functional analog of senA(shET2). Although highly attenuated, CVD1208 is given at very high doses (109) which can induce fever in 10% of subjects.
U.S. Pat. No. 6,759,241 is directed towards an approach involving the msbB2 gene in live vaccine development. The msbB2 gene encodes a fatty acyl transferase enzyme that adds fatty acid residues to the lipid A portion of the bacterial LPS molecule. The lipid A portion constitutes an endotoxin that is responsible for the potent inflammatory activity and pyrogenic properties of bacterial LPS. In an E. coli background, a mutation in the msbB gene results in a less toxic LPS but, unlike E. coli, Shigella has two msbB genes, the first (msbB1) on the chromosome and the second (msbB2) on the virulence plasmid. Deletion of either msbB gene in a S. flexneri 2a background results in reduced histopathology of Shigella infection in the rabbit ileal loop model and deletion of both msbB genes reduces the pathology even further.
Data from our lab, which was presented at the Gordon Conference in 2006, indicates that loss of both msbB1 and msbB2 genes in a S. flexneri 2a background affects the growth and invasive properties of the bacteria. However, loss of msbB1 or msbB2 gene alone does not compromise the growth, invasiveness, immunogenicity, or the protection generated in an animal model of Shigella infection when compared to infection with the wild-type Shigella strain. Furthermore, loss of either msbB gene resulted in lower levels of proinflammatory cytokines (e.g., IL-1b and MCP-1) in lung washes of mice intranasally infected with the bacteria.
A review article in Expert Review Vaccines 2006, 5:5 discusses several live-attenuated Shigella vaccines with well-defined mutations in specific genes. In these studies, the immune responses obtained with each vaccine strain were compared with data obtained from challenge trials using wild-type Shigella strains. The data from these trials indicated that live attenuated vaccine strains elicited immune responses comparable to individuals challenged with wild-type Shigella strains. Thus, the combination of gene deletions is expected to eliminate reactogenicity while inducing protective immune responses.
An article appearing in Vaccine 2007, 25: 2269-2278 describes a S. Flexneri 2a vaccine candidate, WRSf2G11, that has deletions in the virG(icsA), set, and sen genes. This strain was constructed using the lambda red recombinase system and demonstrated that virG-based Shigella vaccine strains lacking enterotoxin genes show lower levels of reactogenicity without hampering robust immune responses.